Investigations of Hadron Structure using CB-TAPS at the Mainz Microtron

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The objective of this research proposal is to build upon our successful program of measuring polarization observables sensitive to the nucleon polarizabilities by continuing collaborative efforts with the A2 group at the Institute for Nuclear Physics in Mainz, Germany. We will exploit the 1.6-GeV MAMI-C continuous-wave electron beam to generate real photons, both polarized and unpolarized, which are combined with polarized and unpolarized nuclear targets in order to probe the structure of hadrons at intermediate energies. In this energy regime, description of structure and reaction dynamics lies in the transition region between nucleon/meson-based models and QCD-inspired models (or effective theories) based on quark/gluon degrees of freedom. Understanding hadronic structure in this transition region of non-perturbative QCD remains at the forefront of research efforts not only at the Mainz laboratory, but also at JLab in the USA, and other intermediate energy labs.******This project will focus specifically on topics within A2 that make use of the Crystal Ball and TAPS detectors. Over the last nine years in Mainz, Hornidge, Huber, and Sarty have been working on nucleon polarizability experiments in addition to studying threshold pion production. Sarty and Huber joined with Hornidge at MAMI in 2008, and this proposal is the third continuation of the Canadian contributions to the A2 Collaboration that will extend to the end of the decade. The experiments we are working on have not only high scientific impact, but they have very close scientific goals to the past, and planned, JLab projects of Huber and Sarty.******As part of our current project grant, we have taken data on four different Compton scattering polarization observables with the end goal of extracting both the scalar and spin polarizabilities of the proton with small statistical, systematic, and model-dependent errors. Such polarizabilities are fundamental structure quantities, amenable to calculation via various QCD-inspired models and effective theories. In addition to taking more data on three of these four asymmetries, over the next three years we will begin taking data on the neutron, where the polarizabilities are much less well known, and we intend to continue our feasibility studies of timelike virtual Compton scattering on the proton using dilepton production. This will allow us, for the first time, to access the timelike electromagnetic structure of the proton at low momentum transfer.******Finally, work will continue on the development and testing of an active, polarized target to use in both polarizability experiments and threshold pion production. By using such a target we will be able to improve on our double-polarization Compton measurements along with the target and beam-target asymmetry measurements in threshold pion production to test isospin breaking at the few percent level due to the up-down quark mass difference.